It has been recognized that containers fabricated using in-mold labeling processes have higher quality in the labeling surface and in the appearance. Typical in-mold labeling (IML) processes involve (1) locating label inside the blow mold and then (2) blowing the parison in the mold against the confines of the mold. The heat from the parison and the pressure generated by the blow medium activates adhesive on the back of the label to bind the label on the blown container side wall.
Some of the problems normally encountered in the application of the IML methods are:
1. Deformation or bulging of the container side wall caused by the difference of shrinkage between the side wall and the label.
2. Prolonging blow molding cycle time due to the decrease of the overall thermal conductance between the plastic wall and the mold (separated by the label which generally has a lower thermal conductivity).
3. Blistering of the labeled side wall.
4. Increase in the overflow volume.
5. A lower fill point due to the increase in the overflow volume possibly giving the impression to a consumer that the container does not contain the required fluid contents.
It has been discovered that the side wall deformation is mainly caused by the fact that the paper label used in the process shrinks less than the plastic side wall during the cooling process. Another reason for the deformation is the fact that the paper label has a larger modulus of elasticity or is stiffer than the plastic.
In accordance with the invention, an in-molding process of forming plastic containers wherein undesirable label panel deformation is eliminated which comprises controlling the moisture of the label such that the shrinkage of the label matches the shrinkage of the plastic side wall against which the label is placed.